BET Protein Inhibitor JQ1 Attenuates Myc-Amplified MCC Tumor Growth In Vivo

Abstract

Merkel cell carcinoma (MCC) is an aggressive neuroendocrine tumor of the skin currently with no cure. In this study, we have first demonstrated that c-Myc overexpression is common in MCC. By targeting c-Myc, bromodomain inhibitors have demonstrated antitumor efficacy in several preclinical human cancer models. Thus we interrogated the role of c-Myc inhibition in MCC with c-Myc amplification by employing the BET inhibitor JQ1. We have uncovered that c-Myc can be regulated by JQ1 in MCC cells with pathological c-Myc activation. Moreover, JQ1 potently abrogates c-Myc expression in MCC cells and causes marked G1 cell cycle arrest. Mechanistically, JQ1 induced cell cycle arrest coincides with downrgulation of cyclin D1 and upregulation of p21, p27 and p57, whereas JQ1 exerts no effect on apoptosis in MCC cells. Further knockdown of p21, p27 or p57 by shRNA partially protects cells from JQ1 induced cell cycle arrest. Additionally, c-Myc knockdown by shRNA generates significant cell cycle arrest, suggesting that c-Myc overexpression plays a role in MCC pathogenesis. Most importantly, JQ1 significantly attenuates tumor growth in xenograft MCC mouse models. Our results provide initial evidence indicating the potential clinical utility of BET protein inhibitors in the treatment of MCC with pathologic activation of c-Myc.

Disruption of BRD4 at H3K27Ac-enriched enhancer region correlates with decreased c-Myc expression in Merkel cell carcinoma

“…Based on our previous findings, we have selected a total of 5 human MCC tumors in which c-Myc expression was examined by immunoblotting using an antibody against c-Myc. 18 Three tumors (tumors #11, #12 and #16) overexpressed c-Myc, whereas 2 tumors (tumor #10 and #15) had basal c-Myc expression. 18 We analyzed these 5 tumors for relative enrichment of H3K27Ac and BRD4 at the putative c-Myc super-enhancer region.…”

“…13 Interestingly, we and another group have found that pathologic c-Myc amplification is common in MCC. 18,19 Moreover, we have also shown that JQ1 represses tumor growth in xenograft MCC mouse models, suggesting an epigenetic mechanism in regulating key oncogene expressions in MCC. 18 In this study, we sought to investigate the potential role of superenhancers in regulating c-Myc overexpression in MCC.…”

“…18,19 Moreover, we have also shown that JQ1 represses tumor growth in xenograft MCC mouse models, suggesting an epigenetic mechanism in regulating key oncogene expressions in MCC. 18 In this study, we sought to investigate the potential role of superenhancers in regulating c-Myc overexpression in MCC. Taking advantage of chromatin immunoprecipitation coupled with realtime or quantitative PCR (ChIP-qPCR), we examined the co-occupancy of H3K27Ac and BRD4 at the putative c-Myc super-enhancer in MCC.…”

“…21 All four MCC cell lines grew in suspension in culture and expressed characteristic markers of MCC as described previously. 18 Both immunoblotting and quantitative reverse transcription PCR (qRT-PCR) demonstrated a significantly higher expression of c-Myc in MCC-3 and MCC-5 cells as compared to that in MKL-1 and MKL-2 cells (Figs. 1A and B).…”

“…We have previously shown that c-Myc overexpression is common in MCC. 18 Here, we wanted to investigate the mechanism involved in regulating c-Myc expression in MCC. At first, we assessed the relative protein and mRNA levels of c-Myc in 4 primary human MCC cell lines.…”